Recently, as information and communication technology has been rapidly developed, a ubiquitous society based on information and communication technology is being developed.
In order to connect information communication devices anytime anywhere, sensors equipped with a computer chip having a communication function should be installed in all social facilities. Accordingly, supplying power to such devices or sensors is a new challenge. In addition, as the types of mobile devices such as music players such as Bluetooth handsets or iPods as well as mobile phones have rapidly increased, it is necessary for users to take more time and efforts to charge batteries. As a method of solving such problems, wireless power transmission technology has recently attracted attention.
Wireless power transmission or wireless energy transmission refers to technology for wirelessly transmitting electric energy from a transmitter to a receiver using the magnetic induction principle. In 1800s, electric motors or transformers using the electromagnetic induction principle have begun to be used and, thereafter, attempts have been made to radiate electromagnetic waves such as high frequencies, microwaves and lasers to transfer electric energy. Frequently used electric toothbrushes or some wireless shavers are charged using the electromagnetic induction principle.
Up to now, a wireless energy transmission method may be roughly divided into a magnetic induction method, an electromagnetic resonance method and a radio frequency (RF) transmission method of a short-wavelength radio frequency.
The magnetic induction method uses a phenomenon that, when two coils are located adjacent to each other and then current is applied to one coil, a magnetic flux is generated to cause an electromotive force in the other coil, and is rapidly being commercialized in small devices such as mobile phones. The magnetic induction method may transfer power of up to several hundreds of kilowatts (kW) and has high efficiency. However, since a maximum transmission distance is 1 centimeter (cm) or less, a device to be charged should be located adjacent to a charger or the floor.
The electromagnetic resonance method uses an electric field or a magnetic field instead of using electromagnetic waves or current. The electromagnetic resonance method is rarely influenced by electromagnetic waves and thus is advantageously safe for other electronic devices or human bodies. In contrast, this method may be used in a limited distance and space and energy transmission efficiency is somewhat low.
The short-wavelength wireless power transmission method (briefly, referred to as the RF transmission method) takes advantage of the fact that energy may be directly transmitted and received in the form of a radio wave. This technology is a RF wireless power transmission method using a rectenna. The rectenna is a combination of an antenna and a rectifier and means an element for directly converting RF power into DC power. That is, the RF method is technology of converting AC radio waves into DC. Recently, as efficiency of the RF method has been improved, studies into commercialization of the RF method have been actively conducted
Wireless power transmission technology may be used not only in mobile related industries but also in various industries such as IT, railroad and home appliance.
If a conductor which is not a wireless power receiver, that is, a foreign object (FO), is present in a wireless charging area, an electromagnetic signal received from a wireless power transmitter may be induced in the foreign object. For example, the FO may comprise coins, clips, pins, and ballpoint pens.
If an FO is present between a wireless power receiver and a wireless power transmitter, wireless charging efficiency may be significantly lowered, and the temperatures of the wireless power receiver and the wireless power transmitter may increase due to increase in ambient temperature of the FO. If the FO located in the charging area is not quickly removed, power waste may occur and an apparatus may be damaged due to overheating.
Accordingly, accurate detection of the FO on a wireless power transmission path is becoming an important issue in wireless charging technology.
In a wireless charging system according to current Qi, a method based on path loss of transmit power and a method based on change in quality factor value are used as a representative method of detecting a foreign object placed in a charging area.
However, in the case of the foreign object detection method based on path loss of transmit power, when information on an accurate receive power strength is not received from a wireless power receiver, a wireless power transmitter may fail in foreign object detection.
If information on the receive power strength is not normal, the wireless power transmitter may erroneously determine that a foreign object is placed although the foreign object is not placed on the wireless power transmission path, thereby stopping charging.